In a mobile communication network, orthogonal multiple access in which signals do not interfere with each other is widely used for communication between a base station and a user device (e.g., a mobile station). In orthogonal multiple access, different radio resources are allocated to different user devices. Examples of orthogonal multiple access include code division multiple access (CDMA), time division multiple access (TDMA), and orthogonal frequency division multiple access (OFDMA). For example, Long Term Evolution (LTE) standardized by the 3GPP uses OFDMA for downlink communication. In OFDMA, different frequencies are allocated to different user devices.
In recent years, non-orthogonal multiple access (NOMA) has been proposed as a communication scheme between a base station and a user device (e.g., see Patent Document 1). In non-orthogonal multiple access, the same radio resource is allocated to different user devices. More specifically, a single frequency is allocated to different user devices simultaneously. When non-orthogonal multiple access is applied to downlink communication, a base station transmits signals with a high transmission power to a user device having a large path loss, i.e., a user device with a small received SINR (signal-to interference plus noise power ratio) (generally, a user device located at an edge of a cell area), and a base station transmits signals with a low transmission power to a user device having a small path loss, i.e., a user device with a large received SINR (generally, a user device centrally located in a cell area). Therefore, there may be interference between signals received by each user device and signals addressed to other user devices.
In this case, each user device demodulates signals addressed to the user device using power differences. More specifically, each user device first demodulates a signal having the highest received power. Since the demodulated signal is a signal addressed to the user device located closest to an edge of a cell area (more accurately, the user device having the lowest received SINR), the user device located closest to an edge of the cell area (the user device having the lowest received SINR) ends demodulation. Each of the other user devices cancels from the received signal an interference component that is equivalent to the demodulated signal using an interference canceller and demodulates a signal having the second highest received power. Since the demodulated signal is a signal addressed to a user device located second closest to an edge of the cell area (more accurately, the user device having the second lowest received SINR), the user device located second closest to an edge of the cell area (the user device having the second lowest received SINR) ends demodulation. By repeating the demodulation and the cancellation of a signal having a high power in this manner, every user device is able to demodulate a signal addressed to the user device.
By combining non-orthogonal multiple access with orthogonal multiple access, it is possible to increase the capacity of a mobile communication network, compared with when orthogonal multiple access alone is used. In other words, when orthogonal multiple access alone is used, it is not possible to allocate a certain radio resource (e.g., a frequency) simultaneously to multiple user devices. However, when non-orthogonal multiple access is combined with orthogonal multiple access, it is possible to allocate a certain radio resource simultaneously to multiple user devices.
Representative candidates for an interference canceller used in NOMA include the following three interference cancellers (Non-Patent Document 1).
Symbol-Level Interference Canceller (SLIC)
This interference canceller handles interference signals at the symbol level (that is, for each resource element (RE)) and cancels demodulation results of the interference signals.
Codeword-Level IC (CWIC)
This interference canceller is also referred to as a Turbo SIC (successive interference canceller) or a Codeword SIC, and decodes interference signals at the codeword level and cancels decoding results. A Codeword SIC is described for example in Non-Patent Document 2.
Maximum Likelihood (ML)
This interference canceller performs joint estimation of a desired signal and an interference signal at the symbol level (that is, for each resource element (RE)).
In order to improve performance in NOMA, it is desirable that a receiver have a high-accuracy interference canceller. Application of CWIC is therefore desirable. However, in order to improve the accuracy of an interference canceller, an increased amount of information on interference signals will be required. Since a CWIC cancels results of decoding interference signals, a CWIC will require more types of information elements of interference signals than other interference cancellers will. Information required for a CWIC is described in Section 7.5 of Non-Patent Document 1. Since other interference cancellers cancel results of demodulating interference signals, these other interference cancellers will also require various information to demodulate interference signals.
Here, interference signals are data signals that interfere with a desired data signal for a user device and are addressed to other user devices. In LTE, to demodulate or decode a data signal, information will be required that is included in a control signal corresponding to a user device to which the data signal is addressed. Accordingly, an interference canceller needs to decrypt control signals corresponding to other user devices.
Patent Document 1 describes various methods for allowing a mobile station to recognize control information for other mobile stations in a radio communication system using non-orthogonal multiple access.